RESPIRATION OF PLANTS. 645 



The loss of assimilated substance caused by respiration would appear purpose- 

 less if we had only to do with the accumulation of assimilated products ; but these 

 are themselves produced only for the purposes of growth and of all the changes 

 connected with life ; the whole life of the plant consists in complicated movements 

 of the molecules and atoms ; and the forces necessary for these movements are set 

 free by respiration. The oxygen, while decomposing part of the assimilated sub- 

 stance, sets up important chemical changes in the remaining portion, which on their 

 part give rise to diffusion-currents, and these bring into contact substances which 

 again act chemically on one another, and so on. The dependence on respiration of 

 the movements in protoplasm and motile leaves is very evident, since, as has been 

 mentioned, they lose their motility when oxygen is withheld from them. These 

 considerations lead to the conclusion that the respiration of plants has the same 

 essential significance as that of animals ; the chemical equilibrium of the substances 

 is being continually disturbed by it, and the internal movements maintained which 

 make up the life of the plant. Respiration is, it is true, a source of loss of sub- 

 stance; but it is also in addition the perpetual source from which flow the forces 

 necessary to the internal movements ^ 



The combination into carbon dioxide of the inhaled oxygen with a portion 

 of the carbon of the assimilated substance is, like all combustion, accompanied by 

 the production of a corresponding amount of heat; but this only rarely leads to 

 a sensible increase of temperature of the masses of tissue, because respiration, and 

 in consequence the production of heat, is not in general very copious, while the 

 circumstances are very favourable to the loss of heat by the plant. In this respect 

 also plants. may be compared with cold-blooded animals. When an amount of heat 

 is set free in the cells by the process of respiration, it first of all distributes itself 

 over the large mass of water which permeates the cells and the adjoining tissue. In 

 the case of a water-plant the least excess of temperature is at once equalised by the 

 surrounding water ; while in the case of a land-plant evaporation has a powerful 

 cooling effect on the atrial parts, quite independently of the action of the radi- 

 ation of heat which is favoured by the large superficial development of most 

 plants, and especially by their hairiness. With these causes of a rapid loss of 

 heat, it is not surprising that the parts of a plant which are expanded in the 

 air are even colder than it, although their respiration is continually producing small 

 quantities of heat. But if the causes of the loss of heat are removed, it is possible 

 to observe with the thermometer the increase of temperature caused by respiration. 

 This can be done by accumulating rapidly germinating seeds, as is shown m the 



^ [M. Corenwinder, from a series of observations on the maple and lilac, has confirmed the 

 view to a certain extent held by Mohl, that the process of respiration is always gomg on m a plant 

 even when concealed by the greater activity of the decomposition of the carbon ^ •o'^^de by the 

 parts containing chlorophyll. He distinguishes two periods in the vegetative season ^^ ^^^ P^^"^- 

 the first period, when nitrogenous constituents predominate is that ^^-^^^^^f /-^^P^'- """neTo 

 active; the second, when the proportion of carbonaceous substance is relaUvely ^-^^ ' ^J^^^^^^^^^^^^^^ 

 when respiration is comparatively feeble, the carbon dioxide evolved bemg -S-"J^^^^^^^^ 

 taken up by the chlorophyll, decomposed, and the carbon fixed m the process «{j^— ^^^^^ 

 found that the proportion of nitrogenous matter in leaves gradually ^^.'-^-^^'^^^^ ^^';^![^^^^';7 

 aceous matter increases, between auUimn and spring. (See R^vue sc.entihque, Aug. i. 1874.-ED.] 



